1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 39 Plant Responses to Internal and External Signals

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Stimuli and a Stationary Life Plants, being rooted to the ground – Must respond to whatever environmental change comes their way

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings For example, the bending of a grass seedling toward light – Begins with the plant sensing the direction, quantity, and color of the light Figure 39.1

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.1: Signal transduction pathways link signal reception to response Plants have cellular receptors – That they use to detect important changes in their environment For a stimulus to elicit a response – Certain cells must have an appropriate receptor

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A potato left growing in darkness – Will produce shoots that do not appear healthy, and will lack elongated roots These are morphological adaptations for growing in darkness – Collectively referred to as etiolation Figure 39.2a (a) Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots.

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings After the potato is exposed to light – The plant undergoes profound changes called de- etiolation, in which shoots and roots grow normally Figure 39.2b (b) After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The potato’s response to light – Is an example of cell-signal processing Figure 39.3 CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Receptor Relay molecules Activation of cellular responses Hormone or environmental stimulus Plasma membrane

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction Second messengers – Transfer and amplify signals from receptors to proteins that cause specific responses – CA++, cAMP, cGMP, Protein Kinases, etc.

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.4 1 Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 activated Transcription factor 1 NUCLEUS P P Transcription Translation De-etiolation (greening) response proteins Ca 2+ Ca 2+ channel opened Specific protein kinase 2 activated Transcription factor 2 An example of signal transduction in plants 1 The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways. 2 One pathway uses cGMP as a second messenger that activates a specific protein kinase.The other pathway involves an increase in cytoplasmic Ca 2+ that activates another specific protein kinase. 3 Both pathways lead to expression of genes for proteins that function in the de-etiolation (greening) response.

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Response Ultimately, a signal transduction pathway – Leads to a regulation of one or more cellular activities In most cases – These responses to stimulation involve the increased activity of certain enzymes – Through transcription factors or post- translational modifications

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transcriptional/Post-Translational Regulation Transcription factors bind directly to specific regions of DNA – And control the transcription of specific genes Post-translational modification – Involves the activation of existing proteins involved in the signal response

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings De-Etioloation (“Greening”) Proteins Many enzymes that function in certain signal responses are involved in photosynthesis directly – While others are involved in supplying the chemical precursors necessary for chlorophyll production

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli Hormones – Are chemical signals that coordinate the different parts of an organism

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Discovery of Plant Hormones Any growth response – That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism – Is often caused by hormones

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Charles Darwin and his son Francis – Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th century

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.5 In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted. EXPERIMENT In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical. CONCLUSION RESULTS ControlDarwin and Darwin (1880) Boysen-Jensen (1913) Light Shaded side of coleoptile Illuminated side of coleoptile Light Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Light Tip separated by gelatin block Tip separated by mica

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In 1926, Frits Went – Extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin. The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark. Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Control Offset blocks cause curvature RESULTS CONCLUSION In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side. EXPERIMENT Figure 39.6

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Survey of Plant Hormones

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In general, hormones control plant growth and development – By affecting the division, elongation, and differentiation of cells Plant hormones are produced in very low concentrations – But a minute amount can have a profound effect on the growth and development of a plant organ

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin The term auxin – Is used for any chemical substance that promotes cell elongation in different target tissues

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Role of Auxin in Cell Elongation According to a model called the acid growth hypothesis – Proton pumps play a major role in the growth response of cells to auxin

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2OH2O Cell elongation in response to auxin Figure 39.8 1 Auxin increases the activity of proton pumps. 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. 2 The cell wall becomes more acidic. 5 With the cellulose loosened, the cell can elongate. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes.

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lateral and Adventitious Root Formation Auxin – Is involved in the formation and branching of roots

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxins as Herbicides An overdose of auxins – Can kill eudicots

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other Effects of Auxin Auxin affects secondary growth – By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinins Cytokinins (think cytokinesis) – Stimulate cell division – Are produced in actively growing tissues such as roots, embryos, and fruits – Work together with auxin

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Control of Apical Dominance Cytokinins, auxin, and other factors interact in the control of apical dominance – The ability of a terminal bud to suppress development of axillary buds Figure 39.9a Axillary buds

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If the terminal bud is removed – Plants become bushier Figure 39.9b “Stump” after removal of apical bud Lateral branches

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anti-Aging Effects Cytokinins retard the aging of some plant organs – By inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gibberellins Gibberellins have a variety of effects – Such as stem elongation, fruit growth, and seed germination

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stem Elongation Gibberellins stimulate growth of both leaves and stems In stems – Gibberellins stimulate cell elongation and cell division

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Growth In many plants – Both auxin and gibberellins must be present for fruit to set

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gibberellins are used commercially – In the spraying of Thompson seedless grapes Figure 39.10

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings After water is imbibed, the release of gibberellins from the embryo – Signals the seeds to break dormancy and germinate Germination Figure 39.11 2 2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is  -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA  -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brassinosteroids – Are similar to the sex hormones of animals – Induce cell elongation and division

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abscisic Acid Two of the many effects of abscisic acid (ABA) are – Seed dormancy – Drought tolerance

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Precocious germination is observed in maize mutants – That lack a functional transcription factor required for ABA to induce expression of certain genes Figure 39.12 Coleoptile ABA is the primary internal signal – enables plants to withstand drought

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ethylene Plants produce ethylene – In response to stresses such as drought, flooding, mechanical pressure, injury, and infection

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Triple Response to Mechanical Stress Ethylene induces the triple response of etiolated seedlings – inhibition of elongation and stem thickening – enhanced apical hook curvature – horizontal growth (90 degrees turned relative to gravity) – this may be a response associated with the seedling growing into an obstacle Figure 39.13 Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response. CONCLUSION Germinating pea seedlings were placed in the dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene. EXPERIMENT All the treated seedlings exhibited the triple response. Response was greater with increased concentration. RESULTS 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million)

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ethylene-insensitive mutants – Fail to undergo the triple response after exposure to ethylene Figure 39.14a ein mutant

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Apoptosis: Programmed Cell Death A burst of ethylene – Is associated with the programmed destruction of cells, organs, or whole plants

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Leaf Abscission A change in the balance of auxin and ethylene controls leaf abscission (relative ratio) – The process that occurs in autumn when a leaf falls Figure 39.16 0.5 mm Protective layer Abscission layer Stem Petiole

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Ripening A burst of ethylene production in the fruit – Triggers the ripening process

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Systems Biology and Hormone Interactions Interactions between hormones and their signal transduction pathways – Make it difficult to predict what effect a genetic manipulation will have on a plant Systems biology seeks a comprehensive understanding of plants – That will permit successful modeling of plant functions

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.3: Responses to light are critical for plant success Light cues many key events in plant growth and development Effects of light on plant morphology – Are what plant biologists call photomorphogenesis

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants not only detect the presence of light – But also its direction, intensity, and wavelength (color) A graph called an action spectrum – Depicts the relative response of a process to different wavelengths of light

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action spectra – Are useful in the study of any process that depends on light Figure 39.17 Wavelength (nm) 1.0 0.8 0.6 0.2 0 450500550600650700 Light Time = 0 min. Time = 90 min. 0.4 400 Phototropic effectiveness relative to 436 nm Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light. EXPERIMENT The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light. RESULTS CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Research on action spectra and absorption spectra of pigments – Led to the identification of two major classes of light receptors: blue-light photoreceptors and phytochromes

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Blue-Light Photoreceptors Various blue-light photoreceptors – Control hypocotyl elongation, stomatal opening, and phototropism

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes as Photoreceptors Phytochromes – Regulate many of a plant’s responses to light throughout its life

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In the 1930s, scientists at the U.S. Department of Agriculture – Determined the action spectrum for light- induced germination of lettuce seeds Dark (control) Dark

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.18 Dark (control) Dark Red Far-red Red Far-red Red DarkRed Far-red Red Far-red CONCLUSION EXPERIMENT RESULTS During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light. The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right). Red light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible.

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A phytochrome – Is the photoreceptor responsible for the opposing effects of red and far-red light A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity. One domain, which functions as the photoreceptor, is covalently bonded to a nonprotein pigment, or chromophore. Kinase activity. The other domain has protein kinase activity. The photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase. Figure 39.19

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes exist in two photoreversible states – With conversion of P r to P fr triggering many developmental responses Figure 39.20 Synthesis Far-red light Red light Slow conversion in darkness (some plants) Responses: seed germination, control of flowering, etc. Enzymatic destruction P fr PrPr

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes and Shade Avoidance The phytochrome system – Also provides the plant with information about the quality of light In the “shade avoidance” response of a tree – The phytochrome ratio shifts in favor of P r when a tree is shaded – apical dominance increases, flowering altered, etc.

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biological Clocks and Circadian Rhythms Many plant processes – Oscillate during the day

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many legumes – Lower their leaves in the evening and raise them in the morning Figure 39.21 Noon Midnight

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cyclical responses to environmental stimuli are called circadian rhythms – And are approximately 24 hours long – Can be entrained to exactly 24 hours by the day/night cycle

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset – Providing the biological clock with environmental cues – Ratio of Pr to Pfr states of phytochrome protein

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Responses to Seasons Photoperiod, the relative lengths of night and day – Is the environmental stimulus plants use most often to detect the time of year Photoperiodism – Is a physiological response to photoperiod

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Control of Flowering Some developmental processes, including flowering in many species – Requires a certain photoperiod

62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod – Are actually controlled by night length, not day length Figure 39.22 During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants. 24 hours Darkness Flash of light Critical dark period Light (a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light. (b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example).

63. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action spectra and photoreversibility experiments – Show that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod Figure 39.23 A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods. 24 20 16 12 8 4 0 Hours Short-day (long-night) plant Long-day (short-night) plant R R FR R RR R Critical dark period

64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Flowering Hormone? The flowering signal, not yet chemically identified – Is called florigen, and it may be a hormone or a change in relative concentrations of multiple hormones

65. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.24 To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced. EXPERIMENT RESULTS CONCLUSION Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant. Plant subjected to photoperiod that induces flowering Plant subjected to photoperiod that does not induce flowering Graft Time (several weeks)

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meristem Transition and Flowering Whatever combination of environmental cues and internal signals is necessary for flowering to occur – The outcome is the transition of a bud’s meristem from a vegetative to a flowering state

67. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.4: Plants respond to a wide variety of stimuli other than light Because of their immobility – Plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms

68. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gravity Response to gravity – Is known as gravitropism Roots show positive gravitropism Stems show negative gravitropism

69. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants may detect gravity by the settling of statoliths – Specialized plastids containing dense starch grains Figure 39.25a, b Statoliths 20  m (a) (b)

70. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanical Stimuli The term thigmomorphogenesis – Refers to the changes in form that result from mechanical perturbation

71. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rubbing the stems of young plants a couple of times daily – Results in plants that are shorter than controls Figure 39.26

72. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rapid leaf movements in response to mechanical stimulation – Are examples of transmission of electrical impulses called action potentials Figure 39.27a–c (a) Unstimulated (b) Stimulated Side of pulvinus with flaccid cells Side of pulvinus with turgid cells Vein 0.5  m (c) Motor organs Leaflets after stimulation Pulvinus (motor organ) Growth in response to touch – Is called thigmotropism – Occurs in vines and other climbing plants

73. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drought During drought – Plants respond to water deficit by reducing transpiration – Deeper roots continue to grow

74. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flooding Enzymatic destruction of cells – Creates air tubes that help plants survive oxygen deprivation during flooding Figure 39.28a, b Vascular cylinder Air tubes Epidermis 100  m (a) Control root (aerated) (b) Experimental root (nonaerated)

75. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants respond to salt stress by producing solutes tolerated at high concentrations – Keeping the water potential of cells more negative than that of the soil solution Cold stress – Altering lipid composition of membranes Heat stress – Heat-shock proteins

76. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.5: Plants defend themselves against herbivores and pathogens Plants counter external threats – With defense systems that deter herbivory and prevent infection or combat pathogens Herbivory, animals eating plants – Is a stress that plants face in any ecosystem Plants counter excessive herbivory – With physical defenses such as thorns – With chemical defenses such as distasteful or toxic compounds

77. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3 Synthesis and release of volatile attractants 1 Chemical in saliva 1 Wounding 2 Signal transduction pathway Some plants even “recruit” predatory animals – That help defend the plant against specific herbivores Figure 39.29

78. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Defenses Against Pathogens A plant’s first line of defense against infection – Is the physical barrier of the plant’s “skin,” the epidermis and the periderm Once a pathogen invades a plant – The plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spread The second defense system – Is enhanced by the plant’s inherited ability to recognize certain pathogens

79. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf. 4 Before they die, infected cells release a chemical signal, probably salicylic acid. 6 In cells remote from the infection site, the chemical initiates a signal transduction pathway. 5 The signal is distributed to the rest of the plant. 2 This identification step triggers a signal transduction pathway. 1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells. 7Systemic acquired resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days. Signal 7 6 5 4 3 2 1 Avirulent pathogen Signal transduction pathway Hypersensitive response Signal transduction pathway Acquired resistance R-Avr recognition and hypersensitive response Systemic acquired resistance Figure 39.31 Plant Responses to Pathogen Invasions A hypersensitive response against an avirulent pathogen – Seals off the infection and kills both pathogen and host cells in the region of the infection

80. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Systemic Acquired Resistance Systemic acquired resistance (SAR) – Is a set of generalized defense responses in organs distant from the original site of infection – Protects against a diversity of pathogens, briefly – Is triggered by the signal molecule salicylic acid